The present inventions relate generally to antenna devices, and more particularly to internal multi-band slot antennas for mobile communication devices and other compact antenna applications.
Dual band antennas are used widely in mobile telephones to accommodate different communication standards. Known external dual band antennas, also referred to as stubby antennas, however, tend to exhibit a high Specific Absorption Rate (SAR) compared to other conventional antennas. Additionally, external and retractable antennas are exposed outside the telephone housing, which is inconvenient for the user. Internal antennas have been proposed to replace external and retractable antennas, but conventional internal antenna designs have do not provide adequate bandwidth, especially for dual mode applications.
Patch micro-strip antennas are considered advantageous in several ways because of their compact lightweight structure, which is relatively easy to fabricate and produce with precise printed circuit techniques capable of integration on printed circuit boards. It is desirable in some applications to provide thin antennas capable of operating in multiple bands having the advantages associated with patch antennas, but prior attempts have been unsuccessful. Additionally, known internal patch antennas tend to have a narrow bandwidth, unless a thick dielectric substrate is employed, but the resulting thickness limits use of the antennas in many applications, particularly in handheld mobile communication devices with severe space and weight constraints.
Conventional patch antennas have natural resonant frequencies or modes for RF and microwave applications. However, there are shortcomings when using natural modes for antenna designs. Natural modes are dependent on the shape and size of the patch. Once the dimensions of the antenna are fixed, the resonant frequencies are also fixed. If the size of the antenna is such that the first mode matches the GSM (900 MHZ) frequency, then the second mode will resonate at its third harmonic, 2700 MHZ, which is not recommended for the DCS (1800 MHZ) frequency. Additionally, to generate natural mode resonant frequencies, the size of the antenna must be relatively large. For example, a 900 MHZ rectangular patch antenna is approximately 12 cm when using a half wavelength patch technique. These large dimensions however are unacceptable for most modern cellular telephone devices, which often require that the antenna be less than approximately 4 cm in length.
Slot antennas may also be implemented in a metal planar surface by providing a gap or a slot in the radiating element. Simple resonant slot antenna geometries include half wavelength and quarter wavelength slot antennas, which are provided with a closed-ended slot or an open-ended slot in the radiating element, respectively. Slot antennas, and conventional patch micro-strip antennas, include a dielectric between the radiating element and a conductive ground plane, with the slot antenna driven differentially from an excitation port, which includes an electrical signal feed point. Slot antennas however also tend to have relatively narrow bandwidths.
The conventional planar inverted F antenna (PIFA) includes a planar radiating element and a ground conductor, as discussed in connection with patch microstrip and slot antenna structures. In the inverted F antenna, the radiating element and the ground conductor are parallel flat conductive surfaces with a feed point and a short circuit end, which resonates with an electric wave at a particular frequency, depending on the length of the radiating conductor. Known PIFA antennas have limitations and generally are not suitable for multi-mode and space limited applications. The conventional PIFA antenna is a quarter wavelength long. The specified frequency generally dictates the length or size of the antenna. If one wants to tune the resonating frequency for another application, the size or some other attribute of the antenna, like the dielectric, must be changed.